This is a bit late but I have a warning about sub-aperture, internal D-ERFs:
There is a fiercely hot, refocused and unfiltered
, reflected Solar image well in front of
my 6" f/8 [120/10] objective.
I'm using a 90mm internal Baader D-ERF with PST H-a filtration in an old CR150HD donor.
Fortunately, my head has usually blocked the sun's light when I happened to glance into the objective while pointing at the Sun.
But an unwary hand waved in front of the objective would probably be burnt since it can't eclipse enough incoming sunlight.
A long dewshield might be a very good idea where the public has even "accidental" access to such an instrument.
An amateur solar observer, with children wandering about, should be very aware of this very real danger.
A small child's head would still allow annular sunlight to fall on a larger objective with potentially tragic consequences.
Most of a lens's collecting area is in the outer zones.
A full aperture D-ERF filter, fixed securely in front of the objective, would minimize the risk down to "normal strength" reflected sunlight.
The reflected glare would usually be enough to cause a person, or child, to desist from their "Darwin Awards" curiosity.
However, a reflected and re-focused image, from an internal D-ERF, might not give the "accidental" viewer any chance at all
to withdraw before damaging their sight permanently.
And yes, you can easily set light to a strip of wood held a few inches in front of the objective on my 6" modified refractor!
Fortunately my telescope is very inaccessible and large enough to need a stepladder to reach the objective.
Logic suggests that the larger
D-ERF the further forward of the objective the Sun's re-focused image will be thrown.
I wouldn't be leaving stepladders anywhere near such an instrument when leaving it unattended.!